The problem arises when the hierarchy manager uses a mix of transient states and unmodified states managed by the local item state manager.
When session 1 is saved, the state change of node a is dispatched to session 2. Node "a" is not changed in session 2, so it's new shared state will be used as-is.
It will have parent "b" according to the hierarchy manager in session 2.

The proposed solution is to keep track of the nodes that have been used in building up the path. This has the downside that building up paths becomes more expensive.
I haven't done any performance measurements yet on the impact of the change. If performance is an issue, perhaps the implementation can be limited to the case that the consistencyCheckEnabled flag is set.